Liner Question

[tryin.not.to.burn.the.house.down]

I did a posting on this once before last week, but I have some new information. Apparently my stainless steel liner that runs through my existing coal burning flu had a gap in it. Come to find out, the piece that connects the thimble to the elbow had come loose. When I talked with the guy that installed and repaired it and he said they were not required by the manufacturer to put screws in the liner to secure the pieces. He explained that the pieces serve as a sleeve into to one another and it went in 4 inches and that should be enough to secure it together. He blames the company that cleaned our pipe for making it come loose.
Is this a load of crap?
How can this truly be safe when the thing is just held together by tension rather than screws???

 

[DAKSY]

Technically, he may be correct. The manufacturer (& the testing authority) determines the installation of his product.
Common sense tells us otherwise, tho, & the installer should've used AT LEAST 3 fasteners at each joint.